1. Field of the Invention
This invention relates to a novel phosphoric acid anodized aluminum support for use in a lithographic printing plate and to a method for the preparation of such support and to a lithographic printing plate comprising the support exhibiting improved abrasion resistance.
2. Description of the Prior Art
An important property of any lithographic surface is the resistance which it exhibits to abrasion in the background or non-image areas. Poor abrasion resistance manifests itself during the printing operation as a gradual wearing away of the non-image surface or as tracks around the plate in the direction of rotation caused by particles imbedded in press rollers.
It is known to prepare lithographic printing plates having good abrasion resistance having a sulfuric acid anodized aluminum support. The sulfuric acid anodized support, however, having thick cell walls and fine pore diameters, does not provide a surface sufficiently porous to achieve adequate adhesion.
U.S. Pat. No. 3,511,661 discloses a lithographic printing plate comprising a phosporic acid anodized aluminum surface. The aluminum surface stratum comprises a cellular pattern of aluminum oxide consisting of cells with porous openings about 20.times.10.sup.-9 m-75.times.10.sup.-9 m in average diameter, thus providing a surface sufficiently porous to achieve good adhesion. The surface stratum comprises about 10 to 200 mg/m.sup.2 aluminum phosphate.
U.S. Pat. No. 4,229,266 relates to the use of a mixture of sulfuric acid and phosphoric acid in forming the anodic layer of a lithographic printing plate. According to this patent, only relatively thin layers are obtained when phosphoric acid alone is used as the electrolyte in the anodizing process, due to the strong redissolving capacity of phosphoric acid towards aluminum oxide, and this is said to result in inferior abrasion resistance for phosphoric acid anodized layers.
Thus, there is a need for lithographic printing plates exhibiting improved abrasion resistance yet having a support surface sufficiently porous to achieve adequate adhesion.